Role of the nuclear envelope in physiological and pathological conditions

The nuclear envelope serves as a selective barrier separating the nucleus from the cytoplasm, thereby ensuring nuclear compartmentalization. During “open” mitosis, the nuclear envelope disassembles to allow chromosome segregation and subsequently reassembles around the daughter nuclei, a process that requires coordinated membrane remodeling mediated by the ESCRT machinery. It is now known that ESCRT-III participates in this process, whereas it remains unclear whether ESCRT-I and ESCRT-II proteins are involved. AKTIP, a recently identified ESCRT-I-associated protein, plays a regulatory role in cytokinesis, where it localizes at the midbody and controls ESCRT-III recruitment and organization. Moreover, AKTIP localizes at the nuclear rim in both interphase and postmitotic cells, exhibiting a characteristic punctate, perinuclear pattern. Based on this evidence, we explored whether AKTIP contributes to nuclear envelope reformation by acting as an upstream factor for ESCRT-III proteins, facilitating their recruitment. We also investigated whether AKTIP plays a role in maintaining postmitotic nuclear integrity. By performing immunofluorescence analyses on control and depleted HeLa cells, obtained through lentiviral transduction, we found that AKTIP localizes at chromatin discs during anaphase. Interestingly, AKTIP colocalizes with the ESCRT-III-associated protein IST1 at anaphase chromatin discs and is required for proper ESCRT-III localization in this stage. During prometaphase, AKTIP organizes into a ring-like structure on nascent microtubule spindles, where it is co-present with BAF. Notably, BAF depletion disrupts AKTIP localization during mitosis. Finally, by analyzing cGAS nuclear foci, LAMB1 discontinuities, and micronuclei formation, we observed that AKTIP depletion resulted in a higher frequency of nuclei displaying structural abnormalities and micronuclei formation. These findings indicate that AKTIP represents a key element of the protein network governing ESCRT-III organization. Its activity begins as early as pro-metaphase, suggesting that AKTIP contributes to the early steps of nuclear envelope remodeling. Moreover, its functional interplay with BAF highlights a coordinated mechanism linking chromatin anchoring to membrane reformation. Such observations may have important implications for aging and tumorigenesis, as alterations in nuclear envelope reformation have been linked to genomic instability, cellular senescence, and dysfunctions that promote tumor development and aging-related phenotypes. Background: During “open” mitosis, the nuclear envelope disassembles to allow chromosome segregation and reassembles around daughter nuclei. The Endosomal Sorting Complex Required for Transport (ESCRT) mediates this process. While ESCRT-III proteins drive this process and are recruited by upstream factors such as BAF, the involvement of other ESCRT proteins remains unclear. AKTIP is a novel ESCRT-I-associated protein. It shows similarities with the ESCRT-I TSG101, it localizes at the midbody during abscission together with the ESCRT-I and ESCRT-III components. Intriguingly, AKTIP also localizes at the nuclear rim in interphase cells. Aim: We therefore asked whether AKTIP contributes to nuclear envelope reformation by coordinating ESCRT-III organization and maintaining post-mitotic nuclear integrity. Methods: To investigate the role of AKTIP in nuclear envelope dynamics, HeLa cells were depleted of AKTIP using lentiviral transduction. Immunofluorescence microscopy was used to examine AKTIP localization during mitosis, its co-distribution with BAF and IST1. The effects of AKTIP loss on the distribution of ESCRT-III components IST1 and CHMP2A was assessed. The role of AKTIP in nuclear integrity was also evaluated by analyzing cGAS foci, laminB1 discontinuities, and micronuclei formation. Results: Data obtained during the PhD show that AKTIP localizes at chromatin discs during anaphase, where it colocalizes with the ESCRT-III-associated protein IST1. AKTIP depletion impaired proper ESCRT-III localization at this stage. During pro-metaphase, AKTIP assembled into a ring-like structure on nascent microtubule spindles, colocalizing with BAF; notably, BAF depletion disrupted AKTIP localization during mitosis. Functional analyses revealed that loss of AKTIP led to increased nuclear abnormalities, including laminB1 discontinuities, cGAS-positive foci, and micronuclei formation. Conclusions: Our findings identify AKTIP as a crucial regulator of ESCRT-III organization during nuclear envelope reformation. Its recruitment begins as early as pro-metaphase, likely through a cooperative mechanism with BAF that links chromatin anchoring to membrane remodeling. AKTIP dysfunction compromises nuclear architecture, highlighting its potential relevance in aging and cancer biology.